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June 10, 2015
In Italy, a tunnel project takes on one of Europe’s most geologically complex areas.
Some 90 000 vehicles a day travel the section of the A1 motorway that connects Bologna and Florence in Italy. A new alternative route, part of the Variante di Valico project administered by Autostrade per l’Italia S.p.A., is being constructed to improve travel time and safety. The road traverses the Appenines, a mountain range that includes unstable combinations of gas, rocks and faults. This geological mix is a major challenge for tunneling projects such as the 3.4-kilometer Val di Sambro tunnel, located in the La Quercia-Badia Nuova section of the Variante di Valico route.
“The material that we found when excavating is what we call the ‘chaotic complex,’ consisting of clays and sandstones,” says Fabio di Giancinto, Project Manager and Environmental Officer for Lagaro, a consortium comprising CMB, Consorzio Coop Costruzioni and CFM. Responsible for building 1.8 kilometers of the tunnel, Lagaro uses excavation methods based on prediction, monitoring and deformation response to excavation that employ conservation systems to maintain the structural characteristics of the soil.
Due to the presence of gas pockets, all the machines and equipment used for the construction of the tunnel are equipped with explosion-proof protection systems. Excavation of the tunnel begins with Atlas Copco HB 4100 breakers. Lagaro Foreman Alberto Perretta describes his team’s progression through the rock: “A breaker carries out the removal of the front, from the foot to the back to the core. Then the breaker is replaced by a debris excavation shovel.” Afterward, an excavator is used to excavate the future position of the arch; the arch is opened, and a flashcrete sprayer creates the initial covering of the rock face.
“Given that we are dealing with a particularly fractured face, we opted for a flashcrete with fiber reinforcing,” says Perretta. “This means we do not have to position mesh over the arches later.”
When the flashcrete operation is complete, the arch is covered with three layers of sprayed concrete (shotcrete). Every three arches, struts are put in place to give continuity to the previous arches. They become a single body, counteracting material subsidence, thus preventing any sagging. The waterproofing is composed of two layers, the first made of a nonwoven fabric, the second Perretta says that on average the team constructs three arches in a 24-hour working day divided into three shifts. “Potentially we produce 2 000 cubic meters of material every 12 linear meters,” he says. “After weeks of monitoring rock face blasting times, the technical director of construction, Lorenzo Contin, has developed a monthly production program that has allowed us to significantly speed up execution times.”
The excavation team is using three Altas Copco HB 4100 breakers. The latest models have a significantly improved power/weight ratio, weighing less with greater efficiency. Ennio Pierdicca, Central Italy Sales Area Manager for Altas Copco Construction Tools, explains that using smaller machines reduces investment expense. “Operating costs are generally lower because the hydraulic system used to operate the breaker requires less power and consumes less fuel,” he says.
Dust suppression is an essential task when working in tunnels. Lagaro proposed changes to the breaker’s spray stick to increase its effectiveness. “In place of the spray adapter plate, which for our purposes was too big, we asked for more compact nozzles to be positioned closer to the tool,” Perretta says. “In this way, the spray is more localized.”
Perretta is pleased with Lagaro’s collaboration with Atlas Copco. “The HB 4100 has completed about 200 hours of work to date and has not caused us any problems,” he says. “We are pleased not only with the high quality of the product but also with the after-sales services, which are really flawless.”
Written by Daniela Stasi